MsgUpdateClient domain type does not faithfully represent the IBC protocol messages

[hdevalence]

Bug Summary

#584 broke the representation of IBC protocol messages, by creating a single domain type, MsgUpdateClient, that represents multiple IBC protocol messages (both MsgUpdateClient and MsgSubmitMisbehavior).

The domain types should faithfully represent the IBC protocol messages, rather than forcing downstream clients to re-parse them to figure out what message they actually got.

This also breaks the use of the ibc_proto traits, forcing callers to write code like

let msg = <MsgUpdateClient as ibc_proto::protobuf::Protobuf<
    ibc_proto::ibc::core::client::v1::MsgUpdateClient,
>>::decode(raw_action_bytes)?;

or

<ibc::core::ics02_client::msgs::update_client::MsgUpdateClient as Protobuf<
    ibc_proto::ibc::core::client::v1::MsgUpdateClient,
>>::encode_vec(&msg)

just to encode or decode messages.

[plafer]

Thank you for bringing this up; I think I found a way to not break them in this way and still implement ibc-go's ADR 6 (our original intent). Will submit a PR soon